In recent years, a fuel cell has been attracting lots of attention as a future clean energy, particularly, amid mounting concern over Global warming prevention as part of global environment issues. The fuel cell is a battery which generates direct current electric power by using hydrogen and oxygen, and the fuel cell is exemplified by a solid electrolyte type fuel cell, a molten carbonate type fuel cell, a phosphoric acid type fuel cell, a solid polymer type fuel cell or the like.
The solid polymer type fuel cell is particularly expected on a global scale to be put into practical use as a dispersed power source for emergency in a small business office, a phone switching station or the like, a dispersed power source for household use using city gas as fuel, and a power source for low-emission electric vehicle using hydrogen gas, methanol or gasoline as fuel, because its is easy to start and stop with an operation temperature as low as about 80° C., and has potential for energy efficiency of about 40%.
Conventionally, as a separator material for solid polymer type fuel cell, application of carbon plate material has been investigated. However, the carbon plate material has the problem of an increased manufacturing cost due to the necessity for precise machining to flatten its surface or form a gas flow path as well as the problem of “high crack susceptibility”. Each of these problems is inherent and renders the very commercialization of the fuel cell difficult.
To solve such problems, application of stainless steel as a separator material for a fuel cell has been attempted also with the primary objective of reducing the manufacturing cost, although this trend faces off against the above-mentioned investigation for application of graphite.
However, use of stainless steel in as-is condition as the separator material is improper since its surface is covered with a passivated film. As measures to this, it has been practiced, as an inexpensive method, to ensure conductivity by including boron in stainless steel and forming a number of projections of boride (boronized compound) on the surface, while gold plating on the surface of stainless steel could be applied.
However, including boron makes the stainless steel hard, and the resulting stainless steel may get cracked during rolling to hinder the rolling or extremely deteriorate the yield of product.
Japanese Patent Application Publication No. 06-246424 discloses, as a method for preventing cracking of B-containing steel in hot rolling from the viewpoint of production, a production method adapted to prevent edge cracking by bonding stainless steels of SUS 316 and SUS 317 to both surfaces of mild steel containing 1% or more boron and serving as a core layer member by means of cast clad.
Japanese Patent Application Publication No. 04-253506 discloses, as a hot rolling process for preventing edge cracking, a method comprising producing an assembled blank composed of an austenitic stainless steel containing 0.3 to 2.0 wt % of B and a steel smaller in flow stress (deformation resistance) than the stainless steel, the steel being welded to the stainless steel so as to cover side edge portions thereof, and finish-rolling the assembled blank at a temperature of (53×B+700)° C. (B: content of B (wt %)) or higher.
Similarly, Japanese Patent Application Publication No. 2001-239364 proposes, for preventing edge cracking of a workpiece to be rolled, a hot working method comprising providing, in hot rolling of an austenitic stainless steel containing 0.3 to 2.5% by mass of B, a protective layer with a thickness of 3 mm or more by overlay weld composed of a stainless steel containing Ni: 4% or less and B: 0.1 to 0.4% on side end faces of the steel.
Japanese Patent Application Publication No. 2004-71319 proposes to bond, between a pair of steel products as corrosion-resistant separator material sheets where electrically conductive non-metallic inclusions are cropped out of the surfaces thereof and exposed to the air, a metallic material higher in ductility than the steel product, so that the superior mechanical property of causing no cracking in press forming in addition to excellent electric contact resistibility with an electrode assembly can be ensured, when a separator composed of such steel is in use.